[C++] How to Create Instance of another class?

[McDonger]

I am new to C++, and I am making a simple RPG game. I have an 'Item' class, which has properties and methods that all items will have. I need to be able to:

a) Inherit from this class to make a class with the item properties and methods, along with new ones

b) Make a new instance of this class in my main class. For example in vb.net:

Class MainClass

dim sword as new ItemClass

End Class

I have tried the following in C++:

new ItemClass sword;

However this gives me the error:

error C2061: syntax error : identifier 'ItemClass'

so it isn't recognising the other class? Thanks for any help :)

[Dav-id]

it is more like

ItemClass *sword;

sword = new ItemClass();

[McDonger]

  whoreman said:

it is more like

ItemClass *sword;

sword = new ItemClass();

Thanks, but I still get the same errors (plus errors saying 'sword' is an undeclared identifier)

The Item.h file is in the same projet, but do I need to reference it in any way at the start of my main class file? (and the code is meant to be in th .h rather than the .cpp file right?) Or when I created this file/ object, what type should I have chosen? (C++ class, or just C++ file?)

Also, for the sake of gaining a better understanding, what does the asterix * mean?

Thanks again

[Kushan]

To create a new instance of a class, you do this:

MyClass NewClassInstance;

If you want to create more than one, you can do this:

MyClass *NewClassInstances = new MyClass[500];

You can essentially treat classes as if they were ints or floats, similar syntax.

In your case, you have created a base item class. You said you wanted to inherit from it in order to create new items. I'm not sure if what you said is what you meant to do, but I'll show you a quick example of what that would be:

//This is your base item class
class Item {

int cost;
char[50] name;
int arbitraryValue;
// etc.

}

class HealingItem : public Item {

// Inherits from Item, can be used just like Item

}

Now when you do this, you can do something rather nifty.

Since all HealingItem's inherit from BaseItem, you can actually use them as such. Here's an example:

Item *MyItem = new HealingItem;

This may seem a bit odd, but in your game you may want to loop through all of the items and rather than having a separate loop for each item type, you can have all of them in one big Item array.

I hope that helps =)

EDIT: The * denotes a pointer. If you haven't looked at C++ pointers yet, I strongly recommend you go do this now as they are very very important.

[Winston]

I think you need to also understand the difference between declaration and definition.

Check this link out:

http://www-ee.eng.hawaii.edu/~tep/EE160/Bo...ion2.1.1.4.html

[Malisk]

About creating instances, note that how you do it depends on if you want to create it on the stack or the heap.

If you create it on the heap, that would be the object* obj = new object(); syntax or object* obj = new object("123") if its constructor took some arguments. If it's rather on the stack (this is pretty common to do too), then it's instead simply object obj; or object obj("123"); if it had a constructor taking some arguments.

Things on the heap needs to be explicitly freed by you at a later point with delete obj;. Creating object instances on the heap is therefore useful if you want to at a later point (like in another function, or to set an object's state as a member variable) access the object, because the compiler won't automatically free it. Creating object instances on the stack make them automatically be freed at the exit of the function they were created in, or the class they were created in, in case of a member variable.

If you create it on the heap, you get a pointer reference to it, and can interact with the object either by typing obj->someFunction(); or by using the "*" dereferencing operator for something like (*obj).someFunction;.

Note that "*" can mean both "this type is a pointer" and "dereference this pointer" depending on how it's used. obj* pointer; is a declaration of a pointer named "pointer", and int i = *pointerToInt; is dereferencing the pointer "pointerToInt" to retrieve the int value it points to and store it in the variable "i".

This "duality" detail puzzled me with C++ for way too long. :p The "->" operator was invented as "syntactic sugar" in C++ so you won't have to do like the ugly above with (*obj).someFunction and it's rarely used like that.

If you create it on the stack, you however don't have a pointer to the object, but can interact with it directly. So then you would instead type obj.someFunction();

Allocating things on the stack is typically faster than allocating on the heap, so it's recommended to do that instead of creating stuff on the heap if you only need to use the variable in a function anyway.

Edited October 14, 2007 by Jugalator

[Endo.]

  Kushan said:

The * denotes a pointer. If you haven't looked at C++ pointers yet, I strongly recommend you go do this now as they are very very important.

I second that. (see Dynamic Memory Allocation).